Taiwan’s president has given a speech in the United Statesthe first time in 15 years that a leader of the island has spoken publicly on American soilprompting an official protest from Beijing.

During a stopover en route to Paraguay, Tsai Ing-wen, whose government refuses to endorse Beijing’s view that Taiwan is part of China, vowed to defend democratic values.

“We will keep our pledge that we are willing to jointly promote regional stability and peace under the principles of national interests, freedom and democracy,” she said on Monday.

China views Taiwan as part of its own territorybe reunified by force if necessaryeven though the two sides split in 1949 after a civil war.

Beijing is always swift to condemn any move that could be interpreted as de facto diplomatic recognition of the government in Taipei and has stepped up pressure on Taiwan since Tsai, of the independence-leaning Democratic Progressive Party (DPP), came to power in 2016.

Tsai made her speech at the Ronald Reagan Presidential Library, whose namesake she praised for his contribution to Taiwan-US relations, including a commitment not to pressure Taipei to negotiate with Beijing.

“Everything was negotiable except two things: our freedom and our future,” she quoted from Reagan’s remarks in her talk, adding that this is also the sentiment of Taiwanese people at the moment.

Her transit in Los Angeles was the most high-profile since former Taiwan president Chen Shui-bian’s 2003 stopover in New York, where he accepted a human rights award and delivered several public speeches.

Washington switched diplomatic recognition to Beijing from Taipei in 1979, but it remains the island’s biggest arms supplier and most important unofficial ally.

Ties have warmed further since Donald Trump came to power, and were further bolstered by the passage this week of the National Defence Authorisation Act, which includes a commitment to support Taiwan militarily.

Last month, the US sent two warships into the Taiwan Strait. That followed a string of military drills staged by Beijing around the island.

China’s Taiwan Affairs Office on Tuesday reiterated its opposition to any attempt to promote Taiwan’s independence when asked to comment on Tsai’s US transit.

“Taiwan is an inalienable part of China. We firmly oppose any attempt to create ‘two Chinas’, ‘one China, one Taiwan’ and ‘Taiwan independence’ in the international arena,” it said in a statement responding to a question from AFP.

The Chinese foreign ministry said it had lodged an official protest with the United States as it recalled that it has always “firmly opposed” the US or other countries with diplomatic relations with China arranging such transits.

The ministry urged Washington to “scrupulously abide by the one China principle” and “not send any wrong signals to the ‘Taiwan independence’ forces”.

Chinese state-run newspaper Global Times accused the US and Taiwan of “shady dealings”, warning that the mainland was capable of giving the Taiwanese authorities “a drastic punishment”.

Tsai’s trip to Paraguay comes as Taiwan seeks to firm up ties with its dwindling band of diplomatic allies, whose number fell to 18 after Burkina Faso and the Dominican Republic switched recognition to Beijing in May.

Under pressure from Beijing, a growing number of international airlines and companies have also edited their websites to refer to the territory as “Taiwan, China” or “Chinese Taipei”.